Systems And Methods For Stabilizing Laser Frequency Based On An Isoclinic Point In The Absorption Spectrum Of A Gas

ABSTRACT

Systems and methods for stabilizing laser frequency based on an isoclinic point of an atomic or molecular medium are provided herein. A system may include: a transmission cell containing a gas and configured to transmit light from the laser, the gas having an absorption spectrum with an isoclinic point; a photodiode generating an output based on an amplitude of transmitted laser light; and circuitry configured to tune the frequency of the laser to the isoclinic point of the absorption spectrum based on the output. The absorption spectrum may have first and second overlapping peaks respectively corresponding to first and second transitions of the gas, the isoclinic point being a saddle point between the first and second peaks. The first and second peaks may have substantially equal amplitude as one another and/or may broaden substantially equally as each other as a function of a physical parameter of the gas.

STATEMENT OF GOVERNMENT INTEREST

This invention was made with government support under Contract No.N66001-09-C-2058 awarded by DARPA. The government has certain rights inthe invention.

FIELD

This application generally relates to systems and methods forstabilizing the frequency of a laser based on a feature in theabsorption spectrum of a gas.

RELATED ART

As is known to those of skill in the art, the frequency of lightgenerated by a laser may change over time. For example, changes intemperature may change the cavity length of the laser, or variations inthe driver current of diode lasers may cause the frequency to changerapidly. Because of such variations, it may be useful to stabilize thefrequency in certain systems.

One system in which frequency stabilization may be useful isultraminiature atomic physics (UAP), in which diode lasers are routinelyused for spectroscopy and/or optical pumping. The chip-scale atomicclock and the chip-scale atomic magnetometer are examples of UAPsystems. Such systems are based on precision spectroscopy aimed atgenerating and accurately probing an atomic interaction over millimeteror smaller scales. One constraint for such systems, however, is thattheir overall size and power may be severely constrained. Moreover, theatomic phenomena may occur under physical conditions that may bring newand sometimes significant dimensions to the atomic physics as comparedto macroscopic laboratory experiments. As such, the stability of thelaser frequency in some circumstances may be critical, not only becausevariations in frequency yield variations in the spectroscopic signals ofinterest, but also because shifts in laser frequency may alter theatoms' energy level structure through the light-shift effect (e.g., theac Stark shift).

In UAP, as well other types of laser-based systems, the laser frequencymay be stabilized by locking the laser to an absorption line of anatomic gas. FIG. 1 schematically illustrates a prior art system 10 forstabilizing the frequency of a laser by locking the laser to an atomicabsorption line. System 10 includes laser 11, gas cell 12, photodiode(PD) 13, and controller circuitry 14. Laser 11 generates light offrequency ω, which is transmitted through gas cell 12. Gas cell 12contains a gas having an absorption spectrum characterized by one ormore peaks corresponding to atomic or molecular transitions. As thelight from laser 11 transmits through cell 12, the light may interactwith one or more transitions of the gas within cell 12. Specifically, ifthe frequency ω_(L) of the laser light is resonant, or near-resonant,with a transition at frequency ω_(A), then that transition (whichappears as a peak in the absorption spectrum of the gas) will at leastpartially absorb the laser light. Such an absorption reduces theirradiance of light transmitted through cell 12. Thus, the closer thelaser frequency ω_(L) is to the absorption peak ω_(A) of the gas, thelower the transmitted irradiance. The photodiode 13 measures theirradiance of the transmitted light and generates an output that is fedinto controller circuitry 14. Based on the output of photodiode 13,controller circuitry 14 determines whether laser frequency ω_(L) ison-resonance with the transition (i.e., ω_(L) is at the peak ω_(A) ofthe absorption line for that transition), and if not, the circuitrysends an appropriate signal to laser 11 to cause the laser to bringω_(L) closer to resonance. Photodiode 13 then measures the irradiance ofthe new frequency ω_(L) of laser light transmitted through cell 12, andthe output is provided to circuitry 14, which may send further signalsto laser 11 to bring ω_(L) still closer to resonance, if required. Inother words, a feedback loop may be used to lock laser 11 to a peak inthe absorption spectrum of the gas within cell 12.

Specifically, the frequency difference between the laser frequency ω_(L)and the frequency absorption peak ω_(A) may be modulated, which causesmodulation in the irradiance measured by photodiode 13. The output ofthe photodiode 13 provides a dispersive-shaped error signal: positivevoltage when ω_(L)>ω_(A), negative voltage when ω_(L)<ω_(A), and zerovoltage when ω_(L)=ω_(A). The error signal is employed in a feedbackloop to lock the laser frequency to a particular value, typically thefrequency where the error signal is zero: ω_(L)=ω_(A). The shape of theerror signal is proportional to the derivative of the absorptionspectrum, so that any physical changes leading to a change in theabsorption frequency ω_(A) will produce frequency shifts in the laserlock frequency ω_(L), as described in greater detail below.

As is familiar to those of ordinary skill in the art, circuitry 14 mayinclude a lock-in amplifier that generates a sinusoidal signal, and acurrent controller that controls the driver current of laser 11. Thelock-in amplifier provides the sinusoidal signal to the currentcontroller, causing the current controller to sinusoidally vary thedriver current of laser 11 about a central current selected to generatefrequency ω_(L). This sinusoidal current variation causes the laserfrequency to vary sinusoidally about frequency ω_(b) typically by arelatively small amount. The sinusoidal variation about frequency ω_(L),causes the irradiance of light transmitted through cell 12 to similarlyvary sinusoidally, as the variation periodically brings the frequencycloser or further from the absorption peak ω_(A). Photodiode 13 recordsthe sinusoidal variations in the irradiance of transmitted light, andthe photodiode output is provided to the lock-in amplifier. The lock-inamplifier carries on-board circuitry that locks to the sinusoidallyvarying output signal. The current controller may then vary the centralcurrent so as to bring the central frequency ω_(L) closer to theabsorption peak ω_(A), which will be detected as a decreased signal atthe photodiode 13. For further details, see the following references,the entire contents of each of which are incorporated by referenceherein: Weel et al., Can. J. Phys. 80, 1449-1458 (2002); Furuta et al.,Appl. Opt. 28(17), 3737-3743 (1989); Akiyama et al., U.S. Pat. No.4,833,681; Telle, U.S. Pat. No. 5,553,087; and Tetu et al., IEEE Trans.Instrum. Meas. 40(2), 191-195 (1991).

In one example, the gas within cell 12 is hydrogen cyanide (H¹³C¹⁴N),the absorption spectrum of which has a series of several spaced peaksbetween about 1525-1565 nm that correspond to rotational-vibrationaltransitions. Laser 11 may be locked to any one of these peaks. Inanother example, the gas within cell 12 is rubidium-87 (Rb⁸⁷), which maybe successfully employed with a significantly lower vapor pressure(e.g., 2×10⁻⁶ torr) than hydrogen cyanide (e.g., about 10 torr),resulting in essentially collisionless conditions. As known to those ofordinary skill in the art, Rb⁸⁷ has four D₁ electronic transitions 20illustrated in FIG. 2A, corresponding to peaks A, B, C, and D in theatomic absorption spectrum 21 of Rb⁸⁷ at 35° C., illustrated in FIG. 2B.Specifically, Rb⁸⁷ has four hyperfine electronic transitions: 5²S_(1/2)(F_(g)=2) to 5²P_(1/2) (F_(e)=1), corresponding to peak A; 5²S_(1/2)(F_(g)=2) to 5²P_(1/2), (F_(e)=2), corresponding to peak B; 5 ²S_(1/2)(F_(g)=1) to 5²P_(1/2) (F_(e)=1), corresponding to peak C; and 5²S_(1/2) (F_(g)=1) to 5²P_(1/2) (F_(e)=2), corresponding to peak D. Notethat although each of the transitions is characterized by a singlefrequency, the corresponding peak in the absorption spectrum is somewhatbroadened because of Doppler broadening, leading to overlap between thepeaks. The x-axis of FIG. 2B, “laser detuning,” refers to the frequencyby which laser 11 may be detuned from the “center of gravity” of theoptical spectrum (3.77×10¹⁴ Hz) to match the absorption feature in thedrawing.

As illustrated in FIG. 2B, the laser is typically locked to frequency23, which corresponds to the 5²S_(1/2) (F_(g)=1) to 5²P_(1/2) (F_(e)=2)electronic transition (the maximum of peak D). This frequency istypically selected because of the four illustrated absorption peaks A-Dpeak D overlaps the least with an adjacent peak (peak C). However, as isfamiliar to those of ordinary skill in the art, the presence ofoverlapping peaks in the absorption spectrum “pulls” the laser frequencyω away from the true center of the desired peak. The breadth of each ofpeaks A-D may vary as a function of the gas temperature, due to Dopplerbroadening, and the relative amplitude of the peaks may also vary as afunction of the gas temperature, because of the nonlinear nature ofresonant absorption, e.g., because of Beer-Lambert exponentialattenuation. As the breadths and/or heights of the different peakschange, the amount of overlap—and thus the amount of pulling—may alsoincrease or decrease with temperature, and as a consequence the peaks ofthe absorption lines may shift with the vapor's temperature. Note thataccording to the Beer-Lambert law, the transmitted irradiance I is equalto I_(o)e^(−NσL), where N is the number density of atoms or molecules inthe gas, σ is the absorption cross-section, and L is the gas length. Theshape of the absorption spectrum (e.g., I versus the laser frequency ω)will mimic the shape of the absorption cross section a for opticallythin gases, where NσL<<1. For optimized laser stabilization systems,where NσL˜1, the detailed shape of the absorption spectrum may deviatefrom the absorption cross section σ, and will depend on N. Specifically,the absorption spectrum will have a width that increases with N.

One way of reducing the change in the locking frequency is sub-Dopplerspectroscopy. In such a technique, the apparent Doppler broadening isreduced by irradiating gas cell 12 with overlapping, counter-propagatinglaser beams. Each of the counter-propagating beams experiences anopposite Doppler shift as the other, canceling out the Dopplerbroadening effect. For further details, see Schawlow, Rev. Mod. Phys.54(3), 697-707 (1982), the entire contents of which are incorporated byreference herein. However, sub-Doppler spectroscopy may not be availableto eliminate all sources of pulling. For example, it may be preferableto use linear absorption spectroscopy, instead of sub-Dopplerspectroscopy, for UAP. Specifically, linear spectroscopy allows forsimplicity (and thereby compactness) of design. Further, without theneed of overlapped, counter-propagating beams these is less sensitivityto microphonics. Additionally, although sub-Doppler spectroscopy mayreduce pulling due to Doppler broadening, it is ineffective to reducepulling resulting from other types of broadening, such as pressurebroadening.

SUMMARY

Embodiments of the invention provide systems and methods for stabilizinglaser frequency based on an isoclinic point in the absorption spectrumof a gas. An isoclinic point is defined to be “[a] wavelength,wavenumber, or frequency at which the first derivative of an absorptionspectrum of a sample does not change upon a chemical reaction orphysical change of the sample.” For many of the systems and methodsdisclosed herein, the isoclinic point is a point in the absorptionspectrum of a gas that falls in between two overlapping absorption peaksof substantially equal amplitude, and which experience substantially thesame broadening as a function of a physical parameter, e.g., as afunction of temperature or pressure. Because the two peaks havesubstantially equal amplitude as one another, the isoclinic point is asaddle point (local minimum) in the region of overlap between the twopeaks. As the peaks are evenly broadened due to a change in the physicalparameter, the frequency of the isoclinic point does not significantlychange, but instead remains at substantially constant frequency,independent of the physical parameter. As such, by locking the laser tothe frequency of the isoclinic point, the laser is significantly lesssusceptible to frequency variations than if the laser were locked to anabsorption peak, as was done in the prior art.

Under one aspect, a system for stabilizing the frequency of atunable-frequency laser includes: a transmission cell containing a gasand configured to transmit light from the laser, the gas having anabsorption spectrum with an isoclinic point; a photodiode configured togenerate an output based on an irradiance of laser light transmittedthrough the cell; and circuitry configured to tune the frequency of thelaser to the isoclinic point of the absorption spectrum based on theoutput.

In some embodiments, the absorption spectrum has first and second peaksrespectively corresponding to first and second transitions of the gas,the first and second peaks overlapping with one another, the isoclinicpoint being a saddle point between the first and second peaks. The firstand second peaks may have substantially equal amplitude as one another.The first and second peaks may broaden substantially equally with eachother as a function of a physical parameter of the gas. The physicalparameter may include temperature or pressure. The gas may include anatomic gas and the first and second transitions may be electronictransitions of atoms in the gas. The atomic gas may, for example,include an alkali selected from the group consisting of Rb⁸⁷, Li⁷, Na²³,K³⁹, and K⁴¹. In one embodiment, a fractional frequency of the isoclinicpoint varies by about 1.0×10⁻¹² or less per degree Celsius.

In some embodiments, the circuitry includes a lock-in amplifierconfigured to receive the output of the photodiode and to generate anerror signal based on the output; and a controller in operablecommunication with the laser and the lock-in amplifier, the controllerconfigured to tune the frequency of the laser so as to minimize theerror signal. The controller may tune the frequency of the laser byadjusting a driver current of the laser.

Under another aspect, a method of stabilizing the frequency of atunable-frequency laser may include transmitting light from the laserthrough a cell containing a gas, the gas having an absorption spectrumwith an isoclinic point; measuring an irradiance of the laser lighttransmitted through the cell; and based on the measured irradiance,tuning the frequency of the laser to the isoclinic point of theabsorption spectrum.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 schematically illustrates a prior art system for locking a laserto an atomic absorption line.

FIG. 2A schematically illustrates the electronic transitions of Rb⁸⁷.

FIG. 2B is a plot of the calculated atomic absorption spectrum of Rb⁸⁷as a function of laser detuning frequency.

FIG. 3 is a flow chart of steps in a method of stabilizing a laserfrequency based on an isoclinic point in an absorption spectrum of agas, according to some embodiments of the present invention.

FIG. 4 is a plot of the calculated atomic absorption spectrum of Rb⁸⁷,as a function of laser detuning frequency, near an isoclinic point,according to some embodiments of the present invention.

FIG. 5 is a plot of the calculated change in frequency as a function oftemperature for peak D in the absorption spectrum of FIG. 2B and for theisoclinic point in the absorption spectrum of FIG. 4, according to someembodiments of the present invention.

FIG. 6 is a plot of the calculated atomic absorption spectrum of amixture of Rb⁸⁷ and Rb⁸⁵ as a function of laser detuning frequency.

FIG. 7A is a plot of the calculated change in the fractional frequencyas a function of temperature for the extremum near the isoclinic pointin the absorption spectrum of FIG. 6 for different values of thefractional abundance of Rb⁸⁵ as compared to Rb⁸⁷, according to someembodiments of the present invention.

FIG. 7B is a plot of the calculated change in the fractional frequencyas a function of temperature for the extremum near the isoclinic pointin the absorption spectrum of FIG. 6 for different values of the laserlinewidth, according to some embodiments of the present invention.

FIG. 8 schematically illustrates an experimental setup used to measurethe change in frequency as a function of temperature for the 4.6 GHzpeak in the absorption spectrum of FIG. 2B and for the isoclinic pointin the absorption spectrum of FIG. 6, according to some embodiments ofthe present invention.

FIG. 9A is a plot of the measured frequency change as a function oftemperature for the 4.6 GHz peak in the absorption spectrum of FIG. 2B,obtained using the experimental setup of FIG. 8.

FIG. 9B is a plot of the measured frequency change as a function oftemperature for the isoclinic point in the absorption spectrum of FIG.6, obtained using the experimental setup of FIG. 8, according to someembodiments of the present invention.

FIG. 10 is a plot of the fractional frequency shift as a function of thetemperature for the 4.6 GHz peak in the absorption spectrum of FIG. 2Band for the isoclinic point in the absorption spectrum of FIG. 6,obtained using the experimental setup of FIG. 8, according to someembodiments of the present invention.

DETAILED DESCRIPTION Overview

Embodiments of the present invention provide systems and methods forstabilizing laser frequency based on an isoclinic point in theabsorption spectrum of a gas. As noted above, prior art systems forstabilizing laser frequency have typically been based on locking thelaser frequency to an absorption peak of a gas. However, such a peak maybe susceptible to pulling caused by lineshape overlap due to Dopplerbroadening, which can vary the frequency of the peak—and the laser—as afunction of temperature. By comparison, the present inventors haverecognized that the absorption spectra of some gases contain isoclinicpoints to which the laser can be locked, and that do not substantiallychange in frequency as a function of physical system parameters, such astemperature or pressure. As used herein, an isoclinic point is definedas “[a] wavelength, wavenumber, or frequency at which the firstderivative of an absorption spectrum of a sample does not change upon achemical reaction or physical change of the sample.” Such a definitionis the same as that defined by the International Union of Pure andApplied Chemistry (IUPAC), see Compendium of Chemical Terminology,2^(nd) ed. (the “Gold Book”), Oxford (1997).

As described below, the inventors have discovered that isoclinic pointsexist for alkali atoms with nuclear spin I equal to 3/2, and that thefrequencies of these isoclinic points are effectively independent of gastemperature. That is, the derivative d ν_(o)/dT at the isoclinic pointis spectroscopically insignificant, where ν_(o) is the frequency at theisoclinic point and T is the gas temperature. The sensitivity of thealkali's isoclinic points to laser linewidth and optical pumpingefficiency are also discussed further below. The practical conclusion isthat isoclinic points in the spectra of alkali atoms may be usefulreference frequencies for precision spectroscopy or laser-frequencystabilization, effectively eliminating slow temperature variations as asource of spectroscopic signal instability in UAP or other applicationsin which it is desirable to lock a laser to a frequency. It should beappreciated that although the discussion below is primarily directed togases containing alkali atoms, that other types of atoms or molecules,including those in non-gas states, may also have absorption spectra withisoclinic points to which a laser can be locked. As such, the presentinvention applies to any system having an isoclinic point.

FIG. 3 is a flow chart of steps in an illustrative method 30 ofstabilizing the frequency of a tunable-frequency laser, according tosome embodiments of the present invention. First, light of frequency ωfrom the laser is transmitted through a cell containing an atomic gas,the gas having an absorption spectrum with an isoclinic point (32). Forexample, the absorption spectrum may include first and second peaks thatrespectively correspond to first and second transitions of the gas, andthat may overlap with one another. The isoclinic point may be a saddlepoint between the first and second peaks, e.g., positioned midwaybetween the two peaks, at a local minimum in the absorption spectrum.The first and second peaks may have substantially equal amplitude as oneanother, and may broaden substantially equally as one another as afunction of a physical parameter of the gas (e.g., temperature orpressure). In one illustrative example, the gas includes an alkaliatomic gas having a nuclear spin of 3/2, e.g., Rb⁸⁷, Li⁷, Na²³, K³⁹, orK⁴¹. In such examples, the first and second peaks may correspond to the5²S_(1/2) (F_(g)=2) to 5²P_(1/2) (F_(e)=1) and the 5²S_(1/2) (F_(g)=2)to 5²P_(1/2) (F_(e)=2) electronic transitions of the gas. However, itshould be understood that any material (including gases, solids,liquids, and plasmas) having an isoclinic point may be used.

In one embodiment, the gas is Rb⁸⁷. FIG. 4 is a plot of the calculatedabsorption spectrum of Rb⁸⁷ in the region of absorption peaks A and B ata temperature of 35° C. Similar to FIG. 2B described above, thefrequency of the center of gravity of FIG. 4 is set to zero. Asdescribed in greater detail below, peaks A and B of Rb⁸⁷ correspond to5²S_(1/2) (F_(g)=2) to 5²P_(1/2) (F_(e)=1) and the 5²S_(1/2) (F_(g)=2)to 5²P_(1/2) (F_(e)=2) transitions, overlap with one another due toDoppler broadening, have equal amplitude as one another, and broadensubstantially equally as one another as a function of temperature, againdue to Doppler broadening. As such, the sum 41 of the two peaks has anisoclinic point 42 that is positioned equidistant between peaks A and B,and is a local minimum between the peaks. If the temperature of the gasfluctuates, then the breadth of peaks A and B may vary because ofDoppler broadening. However, the lateral (frequency) position ofisoclinic point 42 will remain substantially the same because thelateral frequency shift from each of the two peaks will cancel eachother out. Instead, only a vertical shift in the position of isoclinicpoint 42 will occur. As the frequency of isoclinic point 42 issubstantially insensitive to temperature, it can be used as a laser lockfrequency to provide significantly greater stability than other pointsin the absorption spectrum. For example, as described above with respectto FIG. 2B, if the maximum peak D were instead selected for the laserlock frequency, then if the temperature of the gas fluctuates, causing achange in the breadth and/or amplitude of peaks C and D, then thevarying amount of overlap between peaks C and D would pull the laserlock frequency away from that maximum, causing instability in the laserlock frequency.

FIG. 5 is a plot of the temperature sensitivity 51 of the maximum ofpeak D illustrated in FIG. 2B, and the temperature sensitivity 52 ofisoclinic point 42 illustrated in FIG. 4. As can be seen, the change infrequency with temperature of the maximum of peak D is several orders ofmagnitude higher than that of isoclinic point 42. As discussed ingreater detail below, the presence of residual Rb⁸⁵ (here, 1% Rb⁸⁵) maycause a small amount of pulling away from isoclinic point 42, leading tosome variation in frequency as a function of temperature. However, evenwith such variation, the selection of the isoclinic point for the laserlock frequency offers a substantial improvement in performance over theselection of an absorption peak maximum, such as that of peak Dillustrated in FIG. 2B.

The laser 11 and cell 12 may be arranged substantially as illustrated inFIG. 1. The laser may be any suitable laser, including a continuous-wave(CW) or pulsed laser, the frequency of which is tunable via any suitablemechanism. For example, the laser may be a diode laser, the frequency ofwhich is tunable by adjusting the temperature of the diodes and/or byadjusting the driver current applied to the diodes. A control module formaking such adjustments may be internal to the laser, or may beexternal. In some embodiments, the frequency of the laser is tuned byadjusting the driver current applied to the diodes, using a suitablecontroller. Preferably, the laser is tunable through a variety offrequencies that correspond to one or more features of the absorptionspectrum of the gas, including the isoclinic point. It should be notedthat because different gases may have isoclinic points in widelydifferent regions of the spectrum, the type of laser 11 and the type ofgas used in cell 12 may be co-selected to respectively provide an outputin the desired frequency range and an isoclinic point to which the laserfrequency may be locked. In one embodiment, laser 11 is a verticalcavity surface-emitting laser (VCSEL), the frequency of which may becontrollably selected by appropriately adjusting the temperature and/ordriver current of the laser.

Referring again to FIG. 3, the irradiance of laser light transmittedthrough the cell is then measured (34). As described above withreference to FIG. 1, the transmitted irradiance depends, among otherthings, on the extent to which the laser light is absorbed bytransitions of the gas. The transmitted irradiance can be measured, forexample, using a suitable photodiode, such as photodiode (PD) 13 in FIG.1.

Then, based on the measured irradiance, the frequency of the laser maybe tuned to the isoclinic point in the absorption spectrum of the gas(36). The general mechanism for such tuning may, for example, be similarto that described above with reference to FIG. 1, e.g., the frequencymay be tuned using a feedback look that includes a lock-in amplifier anda current controller. However, it should be understood that while priorart systems have been directed to tuning the laser to the maximum of anabsorption peak, the systems and methods provided herein instead tunethe laser to a seemingly counterintuitive position in the absorptionspectrum—a local minimum, positioned between two equal-amplitudeabsorption peaks. As described in greater detail below, such a positionis significantly less vulnerable to frequency changes as a function ofphysical parameters, such as temperature or pressure, than is themaximum of a peak.

A brief theoretical overview of the use of isoclinic points to addressthe problem of Doppler broadening will now be provided. Then, thespecific case of isoclinic points in the alkali atoms, in particularRb⁸⁷, is provided, including a consideration of the isoclinic point'ssensitivity to the Rb⁸⁵/Rb⁸⁷ isotope ratio and laser linewidth. Finally,an experimental investigation of the Rb⁸⁷ D₁ transition is discussed,and the effects of optical pumping on the D₁ isoclinic point described.

Isoclinic Points to Address Doppler Broadening

As noted above with respect to FIGS. 2A-2B, peaks corresponding toelectronic transitions may be Doppler broadened, leading to overlap thatmay cause a laser frequency to be pulled away from a desired frequency.To illustrate the problem more quantitatively, consider two neighboringDoppler-broadened transitions, A and B, such as those illustrated inFIG. 2B, where w_(D) is the Doppler-broadened full-width half-maximum(FWHM) of either peak A or B. For a laser of frequency ω_(L) tuned nearthese absorption lines, the irradiance I transmitted by a gas of lengthL can be expressed as:

I(L)=I _(o) exp[−NL[σ _(A)(Δ_(A))+σ_(B)(Δ_(B))]],  (1)

where σ_(J)(Δ_(j)) is the cross section of the J^(th) resonance andΔ_(J) is the detuning from the true resonant frequency of thetransition: Δ_(J)=ω_(L)−ω_(J). Taking the derivative of Eq. (1) withrespect to laser frequency ω_(L) and setting this equal to zero, we findthe extrema in the absorption spectrum:

$\begin{matrix}{\frac{{I(L)}}{\omega_{L}} = {{{- N}\; L\; {^{{- {N{\lbrack{{\sigma_{A}{(\Delta_{A})}} + {\sigma_{B}{(\Delta_{B})}}}\rbrack}}}L}( {\frac{\sigma_{A}}{\omega_{L}} + \frac{\sigma_{B}}{\omega_{L}}} )}} = { 0\Rightarrow( {\frac{\sigma_{A}}{\omega_{L}} + \frac{\sigma_{B}}{\omega_{L}}} )  = 0.}}} & (2)\end{matrix}$

For the case of the extremum near absorption line A, this yields thepeak frequency of the A transition:

$\begin{matrix}{{\omega_{p\; A} = {{\omega_{A} - {{\Delta_{B}( \frac{\sigma_{p\; B}}{\sigma_{p\; A}} )}^{{- 4}{\ln {(2)}}{({\Delta_{B}/w_{D}})}^{2}}}} \cong {\omega_{A} - {( {\omega_{A} - \omega_{B}} )( \frac{\sigma_{p\; B}}{\sigma_{p\; A}} )^{{- 4}{\ln {(2)}}{({{({\omega_{A} - \omega_{B}})}/w_{D}})}^{2}}}}}},} & (3)\end{matrix}$

where σpJ is the peak absorption cross section of the J^(th) transition,and where ω_(pA) is seen to have a temperature dependent shift due tothe temperature sensitivity of the Doppler width. To be clear, ω_(pA) isthe peak frequency of the absorption line A, while ω_(A) is theintrinsic resonant frequency of the transition, e.g., the 5²S_(1/2)(F_(g)=2) to 5²P_(1/2) (F_(e)=1) transition for Rb⁸⁷. In particular, forsmall changes about some reference temperature T_(o), and definingΔ_(AB) as ω_(A)−ω_(B), the peak frequency of the transition will vary as

$\begin{matrix}{\frac{{\delta\omega}_{p\; A}}{\delta \; T} = {{- 4}{\ln (2)}\frac{\Delta_{A\; B}}{T_{o}}( \frac{\sigma_{B}( \Delta_{A\; B} )}{\sigma_{p\; A}} ){( \frac{\Delta_{A\; B}}{w_{D}( T_{o} )} )^{2}.}}} & (4)\end{matrix}$

For absorption line A in FIG. 2B near room temperature this yieldsδω_(pA)/δT≅16 kHz/° C. or in fractional frequency, y, 4.2×10⁻¹¹/° C.(i.e., y≡δω/ω_(o)). This is a relatively large temperature sensitivity,and demonstrates the significance of temperature variations forprecision vapor-phase spectroscopy.

Without wishing to be bound by any theory, the inventors believe thatthe temperature dependence indicated by Eq. (4) may arise from the factthat, near an absorption line's peak, one of the cross-sectionderivatives becomes effectively independent of temperature while theother retains its Doppler-broadening temperature sensitivity.Conversely, near the midpoint between the two resonances, bothderivatives are temperature dependent. In particular, if ω_(m) isdefined to be the frequency corresponding to the local extremum near themidpoint, then for reasonably well-resolved, Doppler-broadenedabsorption lines,

$\begin{matrix}{\omega_{m} \cong {( \frac{\omega_{A} + \omega_{B}}{2} ) - {\frac{( {\sigma_{p\; A} - \sigma_{p\; B}} )}{( {\sigma_{p\; A} + \sigma_{p\; B}} )}{\frac{\Delta_{A\; B}w_{D}^{2}}{{4{\ln (2)}\Delta_{A\; B}^{2}} - {2w_{D}^{2}}}.}}}} & (5)\end{matrix}$

In this case, the temperature dependence of the extremum only ariseswhen the absorption cross-sections of the two transitions are unequal.When σ_(pA) equals σ_(pB), the second term on the right-hand-side of Eq.(5) is identically zero, and the frequency of the local extremum equalsthe intrinsic midpoint frequency of the two transitions independent oftemperature: it is an isoclinic point.

Notwithstanding the above discussion, for precision spectroscopy it isimportant to note that isoclinic points are idealizations. Withoutwishing to be bound by any theory, it is believed that no gas-phaseatomic or molecular spectral feature will ever be insensitive to a“physical change of the sample” to all orders. For example, in realsystems, alkali isotopes often co-exist, and even in a vapor of “pure”Rb⁸⁷ there is always some fractional component of Rb⁸⁵ (e.g., 10%residual Rb⁸⁵) with absorption lines E, F that overlap (albeit slightly)those of Rb⁸⁷, as illustrated in FIG. 6. This overlap implies that Eq.(1) may be augmented with a third absorption cross-section, complicatingthe simple argument leading to Eq. (5). Moreover, because single modelasers may be dominated by white frequency fluctuations, producinglorentzian laser spectra with corresponding long tails, there also maybe an interaction between Rb⁸⁵ contamination and laser linewidth.Finally, an alignment among the ground-state Zeeman sublevels producedby optical pumping may degrade the equality between the A and Bcross-sections, thereby giving the isoclinic point an alternate path totemperature sensitivity. As noted above with respect to FIG. 5, thepresence of residual Rb⁸⁵ may cause the frequency of the isoclinic pointto vary with temperature; however the extent of such variation isexpected to be significantly smaller than that for the maximum of anabsorption peak. Issues associated with residual Rb⁸⁵ are described infurther detail below with reference to FIGS. 6 and 7A-7B.

Isoclinic Points for Alkali Gases

As may be derived from prior art atomic physics theory, the peak crosssection for a D₁ transition in the alkalies (i.e., excited and groundstate electronic angular momenta, J_(e) and J_(g), respectively, equalto ½) originating from the

$F_{g} = {I + \frac{1}{2}}$

ground-state hyperfine manifold (where I is the nucleus's spin angularmomentum quantum number) may be expressed as

$\begin{matrix}{{\sigma_{p}( {F_{g},F_{e}} )} = {{\sigma_{o}\lbrack J_{g} \rbrack}( {1 + \frac{2{\langle{\overset{arrow}{I} \cdot \overset{arrow}{S}}\rangle}}{( {I + 1} )}} )\{ \begin{matrix}{\frac{( {{2I} + 3} )( {I + 1} )}{6( {{2I} + 1} )^{2}};} & {F_{e} = {I + \frac{1}{2}}} \\{\frac{2{I( {I + 1} )}}{3( {{2I} + 1} )^{2}};} & {{F_{e} = {I - \frac{1}{2}}},}\end{matrix} }} & (6)\end{matrix}$

where

{right arrow over (I)}·{right arrow over (S)}

is a measure of ground-state hyperfine polarization (e.g., thepopulation imbalance between the two ground-state hyperfine levels) andσ_(o) is the integrated D₁ absorption cross section (the expressionsherein use the notation [J]≡(2J+1).) Employing the second approximationof Whiting, J. Quant. Spectrosc. Radiat. Transfer 8, 1379-1384 (1968),the entire contents of which are incorporated by reference herein, for aVoigt profile, the functional relationship between σ_(o) and thetransition's oscillator strength, f, can be obtained:

$\begin{matrix}{{\sigma_{o} = \frac{2\pi^{2}r_{o}f\; c}{w_{V}\lbrack {1.065 + {0.447( \frac{w_{L}}{w_{V}} )} + {0.058( \frac{w_{L}}{w_{V}} )^{2}}} \rbrack}},} & (7)\end{matrix}$

where r_(o) is the classical electron radius. The values w_(L), w_(D),and w_(V) correspond to the FWHM of the Lorentzian, Doppler, and Voigtprofiles, respectively, and are related by:

$\begin{matrix}{w_{V} = {\frac{w_{L}}{2} + {\sqrt{\frac{w_{L}^{2}}{4} + w_{D}^{2}}.}}} & (8)\end{matrix}$

Similarly, for the D₁ transition originating from the

$F_{g} = {I - \frac{1}{2}}$

hyperfine manifold,

$\begin{matrix}{{\sigma_{p}( {F_{g},F_{e}} )} = {{\sigma_{o}\lbrack J_{g} \rbrack}( {1 - \frac{2{\langle{\overset{arrow}{I} \cdot \overset{arrow}{S}}\rangle}}{I}} )\{ \begin{matrix}{\frac{2{I( {I + 1} )}}{3( {{2I} + 1} )^{2}};} & {F_{e} = {I + \frac{1}{2}}} \\{\frac{I( {{2I} - 1} )}{6( {{2I} + 1} )^{2}};} & {F_{e} = {I - {\frac{1}{2}.}}}\end{matrix} }} & (9)\end{matrix}$

Writing Whiting's second approximation for the Voigt profile in detail,the frequency dependence of the absorption cross-sections may beexpressed as

$\begin{matrix}{{{\sigma_{F_{g},F_{e}}( \Delta_{J} )} = {{\sigma_{p}( {F_{g},F_{e}} )}\{ {{\lbrack {1 - \frac{w_{L}}{w_{V}}} \rbrack ^{{- 4}{\ln {(2)}}{({\Delta_{J}/w_{V}})}^{2}}} + {\lbrack \frac{w_{L}}{w_{V}} \rbrack ( \frac{1}{1 + {4( {\Delta_{J}/w_{V}} )^{2}}} )} + {{{\frac{1}{62.5}\lbrack {1 - \frac{w_{L}}{w_{V}}} \rbrack}\lbrack \frac{w_{L}}{w_{V}} \rbrack}\{ {^{{- 0.4}{({{\Delta_{J}}/w_{V}})}^{9/4}} - \frac{10}{10 + ( {{\Delta_{J}}/w_{V}} )^{9/4}}} \}}} \}}},} & (10)\end{matrix}$

where the index J corresponds to one of the F_(g)→F_(e) resonancesillustrated in FIGS. 2A-2B.

Note that the two cross sections originating from the

$F_{g} = {I + \frac{1}{2}}$

ground-state hyperfine manifold (expressed by Eq. (6)) will be equalwhen I=3/2, corresponding to the stable alkali isotopes Li⁷, Na²³, K³⁹,K⁴¹, and Rb⁸⁷. Thus, there will be an isoclinic point midway betweenthese transitions. Table I lists the D₁ transition properties of variousalkali isotopes that it is believed would show an isoclinic point at anextremum of the n²S_(1/2) (F_(g)=2)→n²P_(1/2) (F_(e)=1,2) transitions.The temperatures were chosen to produce a vapor density of 10¹⁰ cm⁻³,and Δν_(hfs) corresponds to the hyperfine splitting in the n²P_(1/2)(first resonance) state. Note that only in the case of Rb⁸⁷ will the twoD₁ transitions be resolved relative to the Doppler width, which is givenin the last column. Of these isotopes, as shown in Table I, Rb⁸⁷produces the largest vapor densities at the lowest temperatures, whichmay be particularly useful for UAP applications. Conversely, the twopeak cross sections originating from the

$F_{g} = {I - \frac{1}{2}}$

ground-state hyperfine manifold (expressed by Eq. (9)) are only equalfor the unphysical case of I=−5/2. Thus, for the D₁ transition of thealkalies, excitation from

$F_{g} = {I - \frac{1}{2}}$

will not yield an isoclinic point at an extremum of the absorption crosssection.

TABLE I 1^(st) Resonance λ Δν_(D), Alkali Abundance D₁, nm Δν_(hfs), MHzT, ° C. MHz Li⁷ 93% 670.8 92 291 2872 Na²³ 100% 589.6 189 114 1494 K³⁹93% 769.9 58 53 806 K⁴¹ 7% 769.9 — 53 786 Rb⁸⁷ 28% 794.8 812 25 500

Note that because of the relatively low natural abundance of Rb⁸⁷, itmay be impracticable to completely isolate that isotope relative toRb⁸⁵. As such, as illustrated in FIG. 6, the absorption spectrum 69 ofRb⁸⁷ gas may contain a small amount of Rb⁸⁵, which exhibits absorptionpeaks E and F. Because the Rb⁸⁵ peak E overlaps slightly with Rb⁸⁷ peakB, variations in the amplitude or breadth of peak E as a function oftemperature may cause the laser lock frequency to pull away fromisoclinic point 42 as temperature varies. Using the above equations, thefrequency of the local extremum midway between the 5²S_(1/2)(F_(g)=2)→5²P_(1/2) (F_(e)=1,2) absorption lines of Rb⁸⁷ (A and B inFIG. 4), i.e., the isoclinic point, may be calculated as a function ofthe relative Rb⁸⁵ concentration. Briefly, the cross section for eachtransition may be computed at a given laser frequency, including theRb⁸⁵ contribution, the cross-sections summed to evaluate the Beer's lawattenuation of the laser, and finally the frequency determined near themidpoint where the derivative of the transmitted laser intensity iszero, i.e., near the isoclinic point. The extent to which the frequencyof this local extremum is insensitive to vapor temperature is a measureof how well the extremum approximates an ideal isoclinic point. As usedherein, the term “isoclinic point” may refer both to an ideal isoclinicpoint, and to a non-ideal isoclinic point of a real gas, such as Rb⁸⁷.

Defining η as the fractional abundance of Rb⁸⁵ in the vapor:η≡N(Rb⁸⁵)/[N(Rb⁸⁵)+N(Rb⁸⁷)], the fractional-frequency change of theextremum (isoclinic point) is illustrated in FIG. 7A as a function ofvapor temperature for η=0.01, 0.03, and 0.1. For comparative purposes,the fractional-frequency change of the local extremum corresponding tothe peak of the 5²S_(1/2) (F_(g)=1)→5²P_(1/2)(F_(e)=2) absorption line(i.e., the maximum of peak D in FIG. 2B) is also illustrated. Of all thetransitions in the Rb⁸⁷ D₁ spectrum, peak D is the most well-isolated,having a relatively small overlap with peak C. Note that, as illustratedin FIG. 7A, even for a Rb⁸⁵ abundance of 10%, the temperaturesensitivity of the local extremum near the midpoint of the A-Btransitions is two orders of magnitude smaller than that of the peakfrequency associated with transition D. Consequently, isotope ratiowould appear to have little effect on the isoclinic nature of themidpoint extremum.

FIG. 7B shows the influence of laser linewidth on the change infractional frequency y as a function of temperature T, δy/δT for the A-Bmidpoint extremum (isoclinic point). Note that even for linewidths of100 MHz, such as may be achieved with vertical cavity surface-emittinglasers (VCSELs) at moderate injection currents above threshold, the A-Bmidpoint extremum acts very much like an ideal isoclinic point becauseδy/δT<10⁻¹²/° C. Moreover, δy/δT for this extremum is significantly lessthan the temperature sensitivity of the D-transition's peak frequencyfor all reasonable laser linewidths.

Experiment

FIG. 8 schematically illustrates an experimental system 80 used tomeasure the temperature dependence of the frequencies of the isoclinicpoint 42 and the maximum of peak D for a gas containing approximately99% Rb⁸⁷ and 1% Rb⁸⁵ and having an absorption spectrum such asillustrated in FIG. 6. System 80 includes vertical cavitysurface-emitting laser (VCSEL) 81, here an aluminum-gallium-arsenide(AlGaAs) continuous-wave laser. Laser 81 is in operable communicationwith laser diode (LD) temperature controller 82 and laser diode (LD)current controller 83. LD temperature controller 82 maintains laser 81at a substantially constant temperature, and LD current controller 83adjusts the driver current provided to laser 81 so as to modify theoutput wavelength ω_(L), of laser 81 as desired. Lock-in amplifiers(LIAs) (1) and (2), denoted in FIG. 8 as elements 84 and 85respectively, generate a modulation signal ν_(m) sin(f_(m)t), whereν_(m) is an amplitude of the modulation signal, f_(m) is a frequency ofthe modulation signal, and t is time. The modulation signal is providedto LD current controller 83, which modulates the driver current of laser81 responsive to the signal, causing the laser frequency to oscillateabout ω_(L) with frequency f_(m), and amplitude ν_(m). Beamsplitter 86splits the modulated laser beam into two beams, which were passedthrough first and second cylindrical, Pyrex transmission cells 87, 88.The first cell 87 is maintained at room temperature, and contains amixture of Rb⁸⁷ and 1 torr of nitrogen (N₂), with up to 2% residual Rb⁸⁵as described in greater detail above. The second cell 88 is maintainedat an elevated temperature, and contains Rb⁸⁷ with up to 1% residualRb⁸⁵. The irradiance of laser light through cells 87, 88 is respectivelydetected with Si photodiodes 89, 90, the outputs of which arerespectively provided to lock-in amplifiers (LIAs) 84, 85. The outputsof LIAs 84, 85 (i.e., the error signals from photodiodes 89, 90) areprovided to a Labview data acquisition (DAQ) module, which generatesplots of the error signals as a function of time for the laser lighttransmitted through cells 87, 88. The output of LIA 85 is also providedto a Proportional-Integral (PI) control module 93 that is in operablecommunication with LD current controller 83 and generates signalsinstructing the current controller to adjust the driver current so as tovary the laser frequency ω_(L) based on the amplitude of the errorsignal from LIA 85, e.g., based on the deviation of ω_(L) from thefrequency of the isoclinic point.

FIG. 9A shows the results of a first experiment in which the laserfrequency ω_(L) was locked to the maximum of absorption peak Dillustrated in FIG. 2B. As mentioned above, this peak would be expectedto be the least affected by temperature variations of the four peaksillustrated in FIG. 2B, because peak D overlaps to a relatively smallextent with absorption peak C and therefore would experience lesspulling as a function of temperature. In this experiment, thetemperature of the Rb-only cell 88 was varied between an initialtemperature of 43° C. (at a time of t=0) and a final temperature of roomtemperature, approximately 25° C. (at a time of approximately t=28minutes). The y-axis in FIG. 9A, “1-Torr Cell Frequency Offset/MHz,”refers to the difference between the error signals output from LIA 84and LIA 85, as determined by Labview DAQ module 92. This difference maybe used as a measure of the frequency of laser 81. Specifically, cell 84acted as a frequency discriminator; because laser 81 excited the sametransition in cells 84, 85, the output of LIA 84 was proportional to thefrequency difference in that absorption feature between the two cells.Note, however that because cell 87 also contained 1 ton of N₂, which isknown to cause a pressure shift in the Rb⁸⁷ D₁ transition ofapproximately 7 MHz/torr, the error signal from this cell was non-zerowhen laser 81 was locked to any of the gas absorption features. As canbe seen in FIG. 9A, the frequency of laser 81 varies by approximately 3MHz over the experimental temperature range. The temperature sensitivityof peak D is estimated to be approximately 200 kHz/° C. over theexperimental temperature range.

The same experiment was repeated, but instead locking the laserfrequency ω_(L) to isoclinic point 42 illustrated in FIG. 4. Thetemperature of cell 88 was again increased to 43° C. and allowed to coolto room temperature. FIG. 9B illustrates the results of this experiment,in which it can be seen that there is no perceptible change in thefrequency of laser 81. Based on these results, the temperaturesensitivity of isoclinic point 42 is estimated to be less than or equalto 25 kHz/° C., significantly lower that that for peak D. Note that theestimated temperature sensitivity of the isoclinic point is limited inthis case by experimental error. Specifically, the calibrated output ofLIA 84 merely provides an upper bound on the temperature sensitivity ofany absorption feature in the D₁ Rb⁸⁷ spectrum, including that of theisoclinic point 42. Calibration of the output of LIA 84 was obtained bytuning the laser frequency and measuring the output of LIA 84 underopen-loop conditions. The temperature sensitivity of isoclinic point 42may be more accurately characterized using additional experiments

For example, a similar experiment as above was used to more accuratelydetermine the temperature sensitivity of both peak D and isoclinic point42. The experiment used the same setup shown in FIG. 8, except cell 87was replaced with a cell identical to cell 88. That is, both cells 87and 88 contained Rb⁸⁷ with up to 1% residual Rb⁸⁵. The temperature ofcell 88 was raised from 25° C. to 31° C. in 2° C. steps, with enoughtime between temperature steps to equilibrate the temperature of cell88. After the equilibration period, the difference between the errorsignals output from LIA 84 and LIA 85 was recorded by Labview DAQ module92 for 100 seconds. This difference may be used as a measure of thefrequency of laser 81 using calibrations obtained under open-loopconditions. This process was repeated for each temperature step. Theresults of this experiment are illustrated in FIG. 10, where the shiftin the fractional frequency (y≡δω/ω_(o)) of laser 81 is shown as afunction of temperature of cell 88. The slope of the data in FIG. 10 canbe used to estimate the temperature sensitivity (δy/δT). For theisoclinic point 42, the temperature sensitivity is estimated to be lessthan or equal to 1×10⁻¹²/° C. For peak D, the temperature sensitivity isestimated to be 4×10⁻¹¹/° C.

Alternative Embodiments

While preferred embodiments of the invention are described herein, itwill be apparent to one skilled in the art that various changes andmodifications may be made. The appended claims are intended to cover allsuch changes and modifications that fall within the true spirit andscope of the invention.

1. A system for stabilizing the frequency of a tunable-frequency laser,the system comprising: a transmission cell containing a gas andconfigured to transmit light from the laser, the gas having anabsorption spectrum including first and second peaks respectivelycorresponding to first and second transitions of the gas, the first andsecond peaks overlapping with one another, a saddle point in the overlapbetween the first and second peaks defining with an isoclinic point ofthe absorption spectrum; a photodiode configured to generate an outputbased on an amplitude of laser light transmitted through the cell; andcircuitry configured to lock the frequency of the laser to the isoclinicpoint of the absorption spectrum based on the output.
 2. (canceled) 3.The system of claim 1, wherein the first and second peaks havesubstantially equal amplitude as one another.
 4. The system of claim 3,wherein the first and second peaks broaden substantially equally as eachother as a function of a physical parameter of the gas.
 5. The system ofclaim 4, wherein the physical parameter comprises temperature orpressure.
 6. The system of claim 1, wherein the gas comprises an atomicgas and wherein the first and second transitions are electronictransitions of atoms in the gas.
 7. The system of claim 6, wherein theatomic gas comprises an alkali selected from the group consisting ofRb⁸⁷, Li⁷, Na²³, K³⁹, and ⁴¹K.
 8. The system of claim 1, wherein afractional frequency of the isoclinic point varies by about 10⁻¹² orless per degree Celsius.
 9. The system of claim 1, wherein the circuitrycomprises: a lock-in amplifier configured to receive the output of thephotodiode and to generate an error signal based on the output; and acontroller in operable communication with the laser and the lock-inamplifier, the controller configured to tune the frequency of the laserso as to minimize the error signal.
 10. The system of claim 9, whereinthe controller tunes the frequency of the laser by adjusting a drivercurrent of the laser.
 11. A method of stabilizing the frequency of atunable-frequency laser, the method comprising: transmitting light fromthe laser through a cell containing a gas, the gas having an absorptionspectrum including first and second peaks respectively corresponding tofirst and second transitions of the gas, the first and second peaksoverlapping with one another, a saddle point in the overlap between thefirst and second peaks defining an isoclinic point of the absorptionspectrum; measuring an amplitude of the laser light transmitted throughthe cell; and based on the measured amplitude, locking the frequency ofthe laser to the isoclinic point of the absorption spectrum. 12.(canceled)
 13. The method of claim 11, wherein the first and secondpeaks have substantially equal amplitude as one another.
 14. The methodof claim 13, wherein the first and second peaks broaden substantiallyequally as each other as a function of a physical parameter of the gas.15. The method of claim 14, wherein the physical parameter comprisestemperature or pressure.
 16. The method of claim 11, wherein the gascomprises an atomic gas and wherein the first and second transitions areelectronic transitions of atoms in the gas.
 17. The method of claim 16,wherein the atomic gas comprises an alkali selected from the groupconsisting of Rb⁸⁷, Li⁷, Na²³, K³⁹, and K⁴¹.
 18. The method of claim 11,wherein a wherein a fractional frequency of the isoclinic point variesby about 10⁻¹² or less per degree Celsius.
 19. The method of claim 11,wherein measuring the amplitude of the laser light comprises receivingthe transmitted laser light with a photodiode and generating an errorsignal based on the output with a lock-in amplifier.
 20. The method ofclaim 19, wherein tuning the frequency of the laser comprises adjustinga driver current of the laser.